The invention relates to a circuit constituting an active RC filter for band-rejection or all-pass applications in the high frequency and microwave regions.
The invention is used to provide integrated circuits for high-frequency and microwave filters which can, for example, be used in one embodiment as a band-rejection filter in frequency doublers to reject the unwanted signal at the fundamental frequency or, in another embodiment, can be used as an all-pass filter in so far as they are adjustable phase shifters at a given frequency or as all-pass filter in so far as they are elements constituting wide-band 90 degrees phase shifters.
As active all-pass filter is already known from the publication by J. TOW in "IEEE Spectrum, December 1969, pp. 64-68", entitled "A Step-by-step Active-filter Design".
This publication describes, inter alia, a version of an all-pass filter having a biquadratic transfer function, which filter is shown in FIG. 8 of said publication. This circuit comprises three operational amplifiers which are arranged in series, this chain being fed back. A fourth operational amplifier takes the sum of the output of the third operational amplifier and the input of the filter circuit.
The operational amplifiers used to form this circuit have a gain which is certainly infinite for low frequencies, but which becomes very weak for high frequencies and microwaves. For that reason, the circuit disclosed in said document has the disadvantage that it cannot be used in these frequency domains.
On the other hand, the feedback chain has the disadvantages that in certain circumstances it causes the circuit to oscillate.
FIG. 6 of said publication also shows an active band-pass filter which has characteristics which are similar and consequently have the same disadvantage.
The circuits disclosed in said publication have moreover the following drawbacks: on the one hand, they are formed by a large number of transistors, which increases manufacturing cost in all cases to a high level, causes a large surface area to be occupied and is disadvantageous for the use in integrated circuits, and has a high power consumption and is also not suitable for the above application. On the other hand, their characteristic frequency is not adjustable. Finally, the capacitors used in this circuit are of a considerable dimension and are the reason that this circuit is not integrable.